8K mode interleaver with odd interleaving only and per OFDM symbol permutation code change in a digital video broadcasting (DVB) standard

ABSTRACT

A data processing apparatus maps input symbols to be communicated onto a predetermined number of sub-carrier signals of an Orthogonal Frequency Division Multiplexed (OFDM) symbol. The data processor includes an interleaver memory which reads-in the predetermined number of data symbols for mapping onto the OFDM sub-carrier signals. The interleaver memory reads-out the data symbols on to the OFDM sub-carriers to effect the mapping, the read-out being in a different order than the read-in, the order being determined from a set of addresses, with the effect that the data symbols are interleaved on to the sub-carrier signals. The set of addresses are generated from an address generator which comprises a linear feedback shift register and a permutation circuit.

CROSS-REFERENCE TO RELATED APPLICATION

The present application is a divisional application of U.S. applicationSer. No. 12/256,095, filed Oct. 22, 2008, the entire contents of whichare incorporated herein by reference. This application also claimspriority to United Kingdom Applications Nos. 0721269.9, filed Oct. 30,2007; 0722645.9, filed Nov. 19, 2007; and 0722728.3, filed Nov. 20,2007.

FIELD OF INVENTION

The present invention relates to data processing apparatus operable tomap input symbols onto sub-carrier signals of Orthogonal FrequencyDivision Multiplexed (OFDM) symbols.

The present invention also relates to data processing apparatus operableto map symbols received from a predetermined number of sub-carriersignals of OFDM symbols into an output symbol stream.

Embodiments of the present invention can provide an OFDMtransmitter/receiver.

BACKGROUND OF THE INVENTION

The Digital Video Broadcasting-Terrestrial standard (DVB-T) utilisesOrthogonal Frequency Division Multiplexing (OFDM) to communicate datarepresenting video images and sound to receivers via a broadcast radiocommunications signal. There are known to be two modes for the DVB-Tstandard which are known as the 2 k and the 8 k mode. The 2 k modeprovides 2048 sub-carriers whereas the 8 k mode provides 8192sub-carriers. Similarly for the Digital Video Broadcasting-Handheldstandard (DVB-H) a 4 k mode has been provided, in which the number ofsub-carriers is 4096.

Error correction coding schemes such as Low-Density ParityCheck/Bose-Chaudhuri-Hocquengham (LDCP/BCH) coding, which have beenproposed for DVB-T2 perform better when noise and degradation of thesymbol values resulting from communication is un-correlated. Terrestrialbroadcast channels may suffer from correlated fading in both the timeand the frequency domains. As such, by separating encoded symbols on todifferent sub-carrier signals of the OFDM symbol by as much as possible,the performance of error correction coding schemes can be increased.Accordingly, in order to improve the integrity of data communicatedusing DVB-T or DVB-H, a symbol interleaver is provided in order tointerleave input data symbols as these symbols are mapped onto thesub-carrier signals of an OFDM symbol. Such a symbol interleavercomprises an interleaver memory and an address generator. Theinterleaver is arranged to read-into the interleaver memory the datasymbols for mapping onto the OFDM sub-carrier signals, and to read-outof the memory the data symbols for the OFDM sub-carriers, the read-outbeing in a different order than the read-in, the order being determinedfrom a set of addresses, which are generated by the address generator.For the 2 k mode and the 8 k mode an arrangement has been disclosed inthe DVB-T standard for generating the addresses to effect the mapping.Likewise for the 4 k mode of DVB-H standard, an arrangement forgenerating addresses for the mapping has been provided and an addressgenerator for implementing this mapping is disclosed in European Patentapplication 04251667.4. The address generator comprises a linear feedback shift register which is operable to generate a pseudo random bitsequence and a permutation circuit. The permutation circuit permutes theorder of the content of the linear feed back shift register in order togenerate an address. The address provides an indication of a memorylocation of the interleaver memory for writing the input data symbolinto or reading the input data symbol out from the interleaver memoryfor mapping onto one of the sub-carrier signal of the OFDM symbol.Similarly, an address generator in the receiver is arranged to generateaddresses of the interleaver memory for writing the received datasymbols into or reading the data symbols out from the interleaver memoryto form an output data stream.

In accordance with a further development of the Digital VideoBroadcasting-Terrestrial standard, known as DVB-T2, there is a desire toimprove the communication of data, and more particularly to provide animproved arrangement for interleaving the data symbols onto thesub-carrier signals of OFDM symbols.

SUMMARY OF INVENTION

According to an aspect of the present invention there is provided a dataprocessing apparatus operable to map input data symbols to becommunicated onto a predetermined number of sub-carrier signals of anOrthogonal Frequency Division Multiplexed (OFDM) symbols. The dataprocessing apparatus comprises an interleaver operable to read-into amemory the predetermined number of data symbols for mapping onto theOFDM sub-carrier signals, and to read-out of the memory the data symbolsfor the OFDM sub-carriers to effect the mapping, the read-out being in adifferent order than the read-in, the order being determined from a setof addresses, with the effect that the data symbols are interleaved onthe sub-carrier signals. The data processing apparatus includes anaddress generator operable to generate the set of addresses, an addressbeing generated for each of the input data symbols for mapping the inputdata symbols onto the sub-carrier signals. The address generatorcomprises a linear feedback shift register including a predeterminednumber of register stages, which are operable to generate apseudo-random bit sequence in accordance with a generator polynomial, apermutation circuit arranged to receive the content of the shiftregister stages and to permute the order of the bits present in theregister stages in accordance with a permutation code to form an addressof one of the OFDM sub-carriers, and a control unit operable incombination with an address check circuit to re-generate an address whena generated address exceeds a predetermined maximum valid address. Thepredetermined maximum valid address is approximately eight thousand, thelinear feedback shift register has twelve register stages with agenerator polynomial for the linear feedback shift register ofR′_(i)[11]=R′_(i-1)[0]⊕R′_(i-1)[1]⊕R′_(i-1)[4]⊕R′_(i-1)[6], and thepermutation order forms, with an additional bit, a thirteen bit address.The data processing apparatus is characterised in that the permutationcircuit is arranged to change the permutation code, which permutes theorder of the bits of the register stages to form the set of addressesfrom one OFDM symbol to another.

Embodiments of the present invention can provide a data processingapparatus operable as a symbol interleaver for mapping data symbols tobe communicated on an OFDM symbol, having substantially eight thousandsub-carrier signals, which can provide an improvement in the integrityof the communicated data. The improvement is provided as a result of achange of the permutation code, which is used to change the order of thebits in the feed back shift register, from one OFDM symbol to another.For example, the permutation code used may be one of a sequence ofdifferent permutation codes which is cycled through, for each of aplurality of OFDM symbols. As a result, an improvement is provided inreducing a possibility that successive or data bits which are close inorder in an input data stream are mapped onto the same sub-carrier of anOFDM symbol, so that the error correction encoding can work moreefficiently.

In one embodiment the number of sub-carrier signals maybe a valuesubstantially between six thousand and eight thousand one hundred andninety two. Furthermore, the OFDM symbol may include pilot sub-carriers,which are arranged to carry known symbols, and the predetermined maximumvalid address may depend on a number of the pilot sub-carrier symbolspresent in the OFDM symbol. As such the 8 k mode can be provided with anefficient symbol interleaver, for example for a DVB standard, such asDVB-T2, DVB-T or DVB-H.

In one example the sequence of different permutation codes forms thethirteen bit address R_(i)[n] for the i-th data symbol from the bitpresent in the n-th register stage R′_(i)[n] in accordance with thepermutation code defined by the table:

R′_(i) bit positions 11 10 9 8 7 6 5 4 3 2 1 0 R_(i) bit positions 5 113 0 10 8 6 9 2 4 1  7.

Although the sequence of permutation codes can include any number ofpermutation codes, in one example there are two permutation codes. Inone example, the two permutation codes are:

R′_(i) bit positions 11 10 9 8 7 6 5 4 3 2 1 0 R_(i) bit positions 5 113 0 10 8 6 9 2 4 1 7

and

R′_(i) bit positions 11 10 9 8 7 6 5 4 3 2 1  0  R_(i) bit positions 810 7 6 0 5 2 1 3 9 4 11.

For example, the approximately eight thousand sub-carriers may beprovided as one of a plurality of operating modes, the approximatelyeight thousand sub-carriers providing half or less than half a maximumnumber of sub-carriers in the OFDM symbols of any of the operatingmodes. The input data symbols may be formed into or regarded as firstsets of input data symbols for mapping onto first OFDM symbols andsecond sets of input data symbols for mapping onto second OFDM symbols.The data processing apparatus may be operable to interleave the inputdata symbols from both first and second sets in accordance with an oddinterleaving process. The odd interleaving process includes writing thefirst sets of input data symbols into a first part of the interleavermemory in accordance with a sequential order of the first sets of inputdata symbols, reading out the first sets of input data symbols from thefirst part of the interleaver memory on to the sub-carrier signals ofthe first OFDM symbols in accordance with an order defined by one of thepermutation codes of the sequence, writing the second set of input datasymbols into a second part of the interleaver memory in accordance witha sequential order of the second sets of input data symbols, and readingout the second sets of input data symbols from the second part of theinterleaver memory on to the sub-carrier signals of the second OFDMsymbols in accordance with an order defined by another of thepermutation codes of the sequence.

The first OFDM symbols may be odd OFDM symbols, and the second OFDMsymbols may be even OFDM symbols.

In some conventional OFDM transmitters and receivers, which operate inaccordance with the 2 k or 8 k modes for DVB-T and the 4 k mode forDVB-H, two symbol interleaving processes are used in the transmitter andthe receiver; one for even OFDM symbols and one for odd OFDM symbols.However, analysis has shown that the interleaving schemes designed forthe 2 k and 8 k symbol interleavers for DVB-T and the 4 k symbolinterleaver for DVB-H work better for odd symbols than for even symbols.Embodiments of the present invention are arranged so that only the oddsymbol interleaving process is used unless the transmitter/receiver isin the mode with the maximum number of sub-carriers. Therefore, when thenumber data symbols which can be carried by the sub-carriers of an OFDMsymbol in one of the plurality of operating modes is less than half ofthe number of data symbols, which can be carried in an operating modewhich proves the most number of data bearing sub-carrier signals perOFDM symbol, then an interleaver of the transmitter and the receiver ofthe OFDM symbols is arranged to interleaver the data symbols of both thefirst and second sets using the odd interleaving process. Since theinterleaver is interleaving the data symbols of both the first andsecond sets of data symbols onto the OFDM symbols using the oddinterleaving process, the interleaver uses different parts of theinterleaver memory to write in and read out the data symbols. Thus,compared with the example in which the interleaver is using the oddinterleaving process and the even interleaving process to interleave thefirst and second sets of data symbols onto successive first and secondOFDM symbols, which utilises the available memory, the amount of memorycapacity used is twice the number of data symbols which can be carriedby an OFDM symbol for the odd only interleaving. This is compared with amemory requirement of one times the number of data symbols, which can becarried in an OFDM symbol in the mode with the most number of datasymbols per OFDM symbol using both the odd and even interleavingprocesses. However, the number of sub-carriers per OFDM symbol for thismaximum operating mode is twice the capacity of the next largest numberof sub-carriers per OFDM symbol for any other operating mode with thenext largest number of sub-carriers per OFDM symbol.

According to some examples therefore, a minimum size of the interleavermemory can be provided in accordance with the maximum number of inputdata symbols which can be carried on the sub-carriers of the OFDMsymbols which are available to carry the input data symbols in any ofthe operating modes.

In some embodiments the operating mode which provides the maximum numberof sub-carriers per OFDM symbol is a 32 k mode. The other modes mayinclude one or more of 1 k, 2 k, 4 k, 8 k and 16 k modes. Thus, as willbe appreciated from the above explanation, in the 32 k mode the odd andeven interleaving processes are used to interleave the data symbols, sothat the size of the interleaver memory can be just enough to accountfor 32 k data symbols. However, for the 16 k mode and any of the othermodes, then the odd interleaving process only is used, so that with the16 k mode an equivalent memory size of 32 k symbols is required, withthe 4 k mode an equivalent memory size of 8 k symbols is required, andwith the 2 k mode an equivalent memory size of 4 k symbols is required.

Various aspects and features of the present invention are defined in theappended claims. Further aspects of the present invention include amethod of mapping input symbols to be communicated onto a predeterminednumber of sub-carrier signals of an Orthogonal Frequency DivisionMultiplexed (OFDM) symbol, as well as a transmitter.

BRIEF DESCRIPTION OF DRAWINGS

Embodiments of the present invention will now be described by way ofexample only with reference to the accompanying drawings, wherein likeparts are provided with corresponding reference numerals, and in which:

FIG. 1 is a schematic block diagram of a Coded OFDM transmitter whichmay be used, for example, with the DVB-T2 standard;

FIG. 2 is a schematic block diagram of parts of the transmitter shown inFIG. 1 in which a symbol mapper and a frame builder illustrate theoperation of an interleaver;

FIG. 3 is a schematic block diagram of the symbol interleaver shown inFIG. 2;

FIG. 4 is a schematic block diagram of an interleaver memory shown inFIG. 3 and the corresponding symbol de-interleaver in the receiver;

FIG. 5 is a schematic block diagram of an address generator shown inFIG. 3 for the 8 k mode;

FIG. 6 is a schematic block diagram of a Coded OFDM receiver which maybe used, for example, with the DVB-T2 standard;

FIG. 7 is a schematic block diagram of a symbol de-interleaver whichappears in FIG. 6;

FIG. 8( a) is diagram illustrating results for an interleaver for evenOFDM symbols and FIG. 8( b) is a diagram illustrating results for oddOFDM symbols; FIGS. 8( a) and 8(b) show plots of the distance at theinterleaver output of sub-carriers that were adjacent at the interleaverinput;

FIG. 9 provides a schematic block diagram of the symbol interleavershown in FIG. 3, illustrating an operating mode in which interleaving isperformed in accordance with an odd interleaving mode only; and

FIG. 10 provides a schematic block diagram of the symbol de-interleavershown in FIG. 7, illustrating the operating mode in which interleavingis performed in accordance with the odd interleaving mode only.

DESCRIPTION OF PREFERRED EMBODIMENTS

The following description is provided to illustrate the operation of asymbol interleaver in accordance with the present technique, although itwill be appreciated that the symbol interleaver can be used with othermodes, other DVB standards and other OFDM systems.

FIG. 1 provides an example block diagram of a Coded OFDM transmitterwhich may be used for example to transmit video images and audio signalsin accordance with the DVB-T2 standard. In FIG. 1 a program sourcegenerates data to be transmitted by the Coded Orthogonal FrequencyDivision Multiplexing (COFDM) transmitter. A video coder 2, and audiocoder 4 and a data coder 6 generate video, audio and other data to betransmitted which are fed to a program multiplexer 10. The output of theprogram multiplexer 10 forms a multiplexed stream with other informationrequired to communicate the video, audio and other data. The multiplexer10 provides a stream on a connecting channel 12. There may be many suchmultiplexed streams which are fed into different branches A, B etc. Forsimplicity, only branch A will be described.

As shown in FIG. 1 a Coded Orthogonal Frequency Division Multiplexing(COFDM) transmitter receives the stream at a multiplexer adaptation andenergy dispersal block 22. The multiplexer adaptation and energydispersal block 22 randomizes the data and feeds the appropriate data toa forward error correction encoder 24 which performs error correctionencoding of the stream. A bit interleaver 26 is provided to interleavethe encoded data bits which for the example of DVB-T2 is the LDCP/BCHencoder output. The output from the bit interleaver 26 is fed to a bitinto constellation mapper 28, which maps groups of bits onto aconstellation point, which is to be used for conveying the encoded databits. The outputs from the bit into constellation mapper 28 areconstellation point labels that represent real and imaginary components.The constellation point labels represent data symbols formed from two ormore bits depending on the modulation scheme used. These will bereferred to as data cells. These data cells are passed through atime-interleaver 30 whose effect is to interleaver data cells resultingfrom multiple LDPC code words.

The data cells are received by a frame builder 32, with data cellsproduced by branch B etc in FIG. 1, via other channels 31. The framebuilder 32 then forms many data cells into sequences to be conveyed onCOFDM symbols, where a COFDM symbol comprises a number of data cells,each data cell being mapped onto one of the sub-carriers. The number ofsub-carriers will depend on the mode of operation of the system, whichmay include one of 1 k, 2 k, 4 k, 8 k, 16 k or 32 k, each of whichprovides a different number of sub-carriers according, for example tothe following table:

Number of Sub-carriers Adapted from DVB-T/H Mode Sub-carriers 1k 756 2k1512 4k 3024 8k 6048 16k  12096 32k  24192

Thus in one example, the number of sub-carriers for the 8 k mode is sixthousand and forty eight. For the DVB-T2 system, the number ofsub-carriers per OFDM symbol can vary depending upon the number of pilotand other reserved carriers. Thus, in DVB-T2, unlike in DVB-T, thenumber of sub-carriers for carrying data is not fixed. Broadcasters canselect one of the operating modes from 1 k, 2 k, 4 k, 8 k, 16 k, 32 keach providing a range of sub-carriers for data per OFDM symbol, themaximum available for each of these modes being 1024, 2048, 4096, 8192,16384, 32768 respectively. In DVB-T2 a physical layer frame is composedof many OFDM symbols. Typically the frame starts with one or morepreamble or P2 OFDM symbols, which are then followed by a number payloadcarrying OFDM symbols. The end of the physical layer frame is marked bya frame closing symbols. For each operating mode, the number ofsub-carriers may be different for each type of symbol. Furthermore, thismay vary for each according to whether bandwidth extension is selected,whether tone reservation is enabled and according to which pilotsub-carrier pattern has been selected. As such a generalisation to aspecific number of sub-carriers per OFDM symbol is difficult. However,the frequency interleaver for each mode can interleave any symbol whosenumber of sub-carriers is smaller than or the same as the maximumavailable number of sub-carriers for the given mode. For example, in the1 k mode, the interleaver would work for symbols with the number ofsub-carriers being less than or equal to 1024 and for 16 k mode, withthe number of sub-carriers being less than or equal to 16384.

The sequence of data cells to be carried in each COFDM symbol is thenpassed to the symbol interleaver 33. The COFDM symbol is then generatedby a COFDM symbol builder block 37 which introduces pilot andsynchronising signals fed from a pilot and embedded signal former 36. AnOFDM modulator 38 then forms the OFDM symbol in the time domain which isfed to a guard insertion processor 40 for generating a guard intervalbetween symbols, and then to a digital to analogue convertor 42 andfinally to an RF amplifier within an RF front end 44 for eventualbroadcast by the COFDM transmitter from an antenna 46.

As explained above, the present invention provides a facility forproviding a quasi-optimal mapping of the data symbols onto the OFDMsub-carrier signals. According to the example technique the symbolinterleaver is provided to effect the optimal mapping of input datasymbols onto COFDM sub-carrier signals in accordance with a permutationcode and generator polynomial, which has been verified by simulationanalysis.

As shown in FIG. 2 a more detailed example illustration of the bit tosymbol constellation mapper 28 and the frame builder 32 is provided toillustrate an example embodiment of the present technique. Data bitsreceived from the bit interleaver 26 via a channel 62 are grouped intosets of bits to be mapped onto a data cell, in accordance with a numberof bits per symbol provided by the modulation scheme. The groups ofbits, which forms a data word, are fed in parallel via data channels 64the a mapping processor 66. The mapping processor 66 then selects one ofthe data symbols, in accordance with a pre-assigned mapping. Theconstellation point, is represented by a real and an imaginary componentthat is provided to the output channel 29 as one of a set of inputs tothe frame builder 32.

The frame builder 32 receives the data cells from the bit toconstellation mapper 28 through channel 29, together with data cellsfrom the other channels 31. After building a frame of many COFDM cellsequences, the cells of each COFDM symbol are then written into aninterleaver memory 100 and read out of the interleaver memory 100 inaccordance with write addresses and read addresses generated by anaddress generator 102. According to the write-in and read-out order,interleaving of the data cells is achieved, by generating appropriateaddresses. The operation of the address generator 102 and theinterleaver memory 100 will be described in more detail shortly withreference to FIGS. 3, 4 and 5. The interleaved data cells are thencombined with pilot and synchronisation symbols received from the pilotand embedded signalling former 36 into an OFDM symbol builder 37, toform the COFDM symbol, which is fed to the OFDM modulator 38 asexplained above.

Interleaver

FIG. 3 provides an example of parts of the symbol interleaver 33, whichillustrates the present technique for interleaving symbols. In FIG. 3the input data cells from the frame builder 32 are written into theinterleaver memory 100. The data cells are written into the interleavermemory 100 according to a write address fed from the address generator102 on channel 104, and read out from the interleaver memory 100according to a read address fed from the address generator 102 on achannel 106. The address generator 102 generates the write address andthe read address as explained below, depending on whether the COFDMsymbol is odd or even, which is identified from a signal fed from achannel 108, and depending on a selected mode, which is identified froma signal fed from a channel 110. As explained, the mode can be one of a1 k mode, 2 k mode, 4 k mode, 8 k mode, 16 k mode or a 32 k mode. Asexplained below, the write address and the read address are generateddifferently for odd and even symbols as explained with reference to FIG.4, which provides an example implementation of the interleaver memory100.

In the example shown in FIG. 4, the interleaver memory is shown tocomprise an upper part 100 illustrating the operation of the interleavermemory in the transmitter and a lower part 340, which illustrates theoperation of the de-interleaver memory in the receiver. The interleaver100 and the de-interleaver 340 are shown together in FIG. 4 in order tofacilitate understanding of their operation. As shown in FIG. 4 arepresentation of the communication between the interleaver 100 and thede-interleaver 340 via other devices and via a transmission channel hasbeen simplified and represented as a section 140 between the interleaver100 and the de-interleaver 340. The operation of the interleaver 100 isdescribed in the following paragraphs:

Although FIG. 4 provides an illustration of only four input data cellsonto an example of four sub-carrier signals of a COFDM symbol, it willbe appreciated that the technique illustrated in FIG. 4 can be extendedto a larger number of sub-carriers such as 756 for the 1 k mode 1512 forthe 2 k mode, 3024 for the 4 k mode and 6048 for the 8 k mode, 12096 forthe 16 k mode and 24192 for the 32 k mode.

The input and output addressing of the interleaver memory 100 shown inFIG. 4 is shown for odd and even symbols. For an even COFDM symbol thedata cells are taken from the input channel and written into theinterleaver memory 124.1 in accordance with a sequence of addresses 120generated for each COFDM symbol by the address generator 102. The writeaddresses are applied for the even symbol so that as illustratedinterleaving is effected by the shuffling of the write-in addresses.Therefore, for each interleaved symbol y(h(q))=y′(q).

For odd symbols the same interleaver memory 124.2 is used. However, asshown in FIG. 4 for the odd symbol the write-in order 132 is in the sameaddress sequence used to read out the previous even symbol 126. Thisfeature allows the odd and even symbol interleaver implementations toonly use one interleaver memory 100 provided the read-out operation fora given address is performed before the write-in operation. The datacells written into the interleaver memory 124.2 during odd symbols arethen read out in a sequence 134 generated by the address generator 102for the next even COFDM symbol and so on. Thus only one address isgenerated per symbol, with the read-in and write-out for the odd/evenCOFDM symbol being performed contemporaneously.

In summary, as represented in FIG. 4, once the set of addresses H(q) hasbeen calculated for all active sub-carriers, the input vector Y′=(y′₀,y′₁, y′₂, . . . y′_(Nmax-1)) is processed to produce the interleavedvector Y=(y₀, y₁, y₂, . . . y_(Nmax-1)) defined by:y _(H(q)) =y′ _(q) for even symbols for q=0, . . . ,N _(max)−1y _(q) =y′ _(H(q)) for odd symbols for q=0, . . . ,N _(max)−1

In other words, for even OFDM symbols the input words are written in apermutated way into a memory and read back in a sequential way, whereasfor odd symbols, they are written sequentially and read back permutated.In the above case, the permutation H(q) is defined by the followingtable:

TABLE 1 permutation for simple case where Nmax = 4 q 0 1 2 3 H(q) 1 3 02

As shown in FIG. 4, the de-interleaver 340 operates to reverse theinterleaving applied by the interleaver 100, by applying the same set ofaddresses as generated by an equivalent address generator, but applyingthe write-in and read-out addresses in reverse. As such, for evensymbols, the write-in addresses 342 are in sequential order, whereas theread out address 344 are provided by the address generator.Correspondingly, for the odd symbols, the write-in order 346 isdetermined from the set of addresses generated by the address generator,whereas read out 348 is in sequential order.

Address Generation for the 8 k mode

A schematic block diagram of the algorithm used to generate thepermutation function H(q) is represented in FIG. 5 for the 8 k mode. InFIG. 5 a linear feed back shift register is formed by twelve shiftregister stages 200, in order to generate an address between 0 and 8191,and an xor-gate 202 which is connected to the stages of the shiftregister 200 in accordance with a generator polynomial. Therefore, inaccordance with the content of the shift register 200 a next bit of theshift register is provided from the output of the xor-gate 202 by xoringthe content of shift register R[0] and register stage R[1]. According tothe generator polynomial a pseudo random bit sequence is generated fromthe content of the shift register 200. However, in order to generate anaddress for the 8 k mode as illustrated, a permutation circuit 210 isprovided which effectively permutes the order of the bits within theshift register 200 from an order R′_(i)[n] to an order R_(i)[n] at theoutput of the permutation circuit 210. Twelve bits from the output ofthe permutation circuit 210 are then fed on a connecting channel 212 towhich is added a most significant bit via a channel 214 which isprovided by a toggle circuit 218. A thirteen bit address is thereforegenerated on channel 212. However, in order to ensure the authenticityof an address, an address check circuit 216 analyses the generatedaddress to determine whether it exceeds the maximum number ofsub-carrier signals. If it does then a control signal is generated andfed via a connecting channel 220 to a control unit 224. If the generatedaddress exceeds the maximum number of carrier signals then this addressis rejected and a new address regenerated for the particular symbol.

In summary an (N_(r)−1) bit word R′_(i) is defined, withN_(r)=log₂M_(max), where M_(max)=8 192 in the 8 k mode, using a LFSR(Linear Feedback Shift Register).

The polynomials used to generate this sequence are as follows:

8 k mode: R′_(i)[11]=R′_(i-1)[0]⊕R′_(i-1)[1]⊕R′_(i-1)[4]⊕R′_(i-1)[6]

where i varies from 0 to M_(max)−1

Once one R′_(i), word has been generated, it goes through a permutationto produce another (N_(r)−1) bit word called R_(i). R_(i) is derivedfrom R′_(i) by the bit permutations given in the table below.

TABLE Bit permutation for the 8k mode R′_(i) bit positions 11 10 9 8 7 65 4 3 2 1 0 R_(i) bit positions 5 11 3 0 10 8 6 9 2 4 1 7

As an example, for the permutation code above this means that for mode 8k, the bit number 11 of R′_(i) is sent in bit position number 5 ofR_(i).

The address H(q) is then derived from R_(i) through the followingequation:

${H(q)} = {{\left( {i\mspace{11mu}{mod}\; 2} \right) \cdot 2^{N_{r} - 1}} + {\sum\limits_{j = 0}^{N_{r} - 2}{{R_{i}(j)} \cdot 2^{j}}}}$

The (i mod2)·2^(N) ^(r) ⁻¹ part of the above equation is represented inFIG. 5 by the toggle block T 218.

An address check is then performed on H(q) to verify that the generatedaddress is within the range of acceptable addresses: if (H(q)<N_(max)),where in one example N_(max)=6048 in the 8 k mode, then the address isvalid. If the address is not valid, the control unit is informed and itwill try to generate a new H(q) by incrementing the index i.

The role of the toggle block is to make sure that we do not generate anaddress exceeding N_(max) twice in a row. In effect, if an exceedingvalue was generated, this means that the MSB (i.e. the toggle bit) ofthe address H(q) was one. So the next value generated will have a MSBset to zero, insuring to produce a valid address.

The following equations sum up the overall behaviour and help tounderstand the loop structure of this algorithm:

q = 0; for  (i = 0; i < M_(ma x); i = i + 1)$\left\{ {{{H(q)} = {{\left( {i\mspace{11mu}{mod}\; 2} \right) \cdot 2^{N_{r} - 1}} + {\sum\limits_{j = 0}^{N_{r} - 2}{{R_{i}(j)} \cdot 2^{j}}}}};{{{if}\mspace{14mu}\left( {{H(q)} < N_{{ma}\; x}} \right)\mspace{14mu} q} = {q + 1}};} \right\}$

As will be explained shortly, in one example of the address generator,the above mentioned permutation code is used for generating addressesfor all OFDM symbols. In another example, the permutation codes may bechanged between symbols, with the effect that a set of permutation codesare cycled through for successive OFDM symbols. To this end, the controllines 108, 110 providing an indication as to whether the OFDM symbol isodd or even and the current mode are used to select the permutationcode. This example mode in which a plurality of permutation codes arecycled through is particularly appropriate for the example in which theodd interleaver only is used, which will be explained later. A signalindicating that a different permutation code should be used is providedvia a control channel 111. In one example the possible permutation codesare pre-stored in the permutation code circuit 210. In another example,the control unit 224 supplies the new permutation code to be used for anOFDM symbol.

Receiver

FIG. 6 provides an example illustration of a receiver which may be usedwith the present technique. As shown in FIG. 6, a COFDM signal isreceived by an antenna 300 and detected by a tuner 302 and convertedinto a digital form by an analogue-to-digital converter 304. A guardinterval removal processor 306 removes the guard interval from areceived COFDM symbol, before the data is recovered from the COFDMsymbol using a Fast Fourier Transform (FFT) processor 308 in combinationwith a channel estimator and correction 310 in co-operation with aembedded-signalling decoding unit 311, in accordance with knowntechniques. The demodulated data is recovered from a mapper 312 and fedto a symbol de-interleaver 314, which operates to effect the reversemapping of the received data symbol to re-generate an output data streamwith the data de-interleaved.

The symbol de-interleaver 314 is formed from a data processing apparatusas shown in FIG. 7 with an interleaver memory 540 and an addressgenerator 542. The interleaver memory is as shown in FIG. 4 and operatesas already explained above to effect de-interleaving by utilizing setsof addresses generated by the address generator 542. The addressgenerator 542 is formed as shown in FIG. 7 and is arranged to generatecorresponding addresses to map the data symbols recovered from eachCOFDM sub-carrier signals into an output data stream.

The remaining parts of the COFDM receiver shown in FIG. 6, including theBit De-Interleaver 316, are provided to effect error correction decoding318 to correct errors and recover an estimate of the source data.

One advantage provided by the present technique for both the receiverand the transmitter is that a symbol interleaver and a symbolde-interleaver operating in the receivers and transmitters can beswitched between the 1 k, 2 k, 4 k, 8 k, 16 k and the 32 k mode bychanging the generator polynomials and the permutation order. Hence theaddress generator 542 shown in FIG. 7 includes an input 544, providingan indication of the mode as well as an input 546 indicating whetherthere are odd/even COFDM symbols. A flexible implementation is therebyprovided because a symbol interleaver and de-interleaver can be formedas shown in FIGS. 3 and 7, with an address generator as illustrated inFIG. 5. The address generator can therefore be adapted to the differentmodes by changing to the generator polynomials and the permutationorders indicated for each of the modes. For example, this can beeffected using a software change. Alternatively, in other embodiments,an embedded signal indicating the mode of the DVB-T2 transmission can bedetected in the receiver in the embedded-signalling processing unit 311and used to configure automatically the symbol de-interleaver inaccordance with the detected mode.

Optimal Use of Odd Interleavers

As shown in FIG. 4, two symbol interleaving processes, one for evenCOFDM symbols and one for odd COFDM symbols allows the amount of memoryused during interleaving to be reduced. In the example shown in FIG. 4,the write in order for the odd symbol is the same as the read out orderfor the even symbol therefore, while an odd symbol is being read fromthe memory, an even symbol can be written to the location just readfrom; subsequently, when that even symbol is read from the memory, thefollowing odd symbol can be written to the location just read from.

The selection of the polynomial generator and the permutation codesexplained above have been identified following simulation analysis ofthe relative performance of the interleaver. The relative performance ofthe interleaver has been evaluated using a relative ability of theinterleaver to separate successive symbols or an “interleaving quality”.The relative measure of the interleaver quality is determined bydefining a distance D (in number of sub-carriers). A criterion C ischosen to identify a number of sub-carriers that are at distance ≦D atthe output of the interleaver that were at distance ≦D at the input ofthe interleaver, the number of sub-carriers for each distance D thenbeing weighted with respect to the relative distance. The criterion C isevaluated for both odd and even COFDM symbols. Minimising C produces asuperior quality interleaver.

$C = {{\sum\limits_{1}^{d = D}{{N_{even}(d)}/d}} + {\sum\limits_{1}^{d = D}{{N_{odd}(d)}/d}}}$

where: N_(even)(d) and N_(odd)(d) are number of sub-carriers in an evenand odd symbol respectively at the output of the interleaver that remainwithin d sub-carrier spacing of each other.

As mentioned above, during an experimental analysis of the performanceof the interleavers (using criterion C as defined above) and for exampleshown in FIG. 8( a) and FIG. 8( b) it has been discovered that theinterleaving schemes designed for the 2 k and 8 k symbol interleaversfor DVB-T and the 4 k symbol interleaver for DVB-H work better for oddsymbols than even symbols. Thus from performance evaluation results ofthe interleavers, for example for the 16 k, as illustrated by FIGS. 8(a) and 8(b) it has been revealed that the odd interleavers work betterthan the even interleavers. This can be seen by comparing FIG. 8( a)which shows results for an interleaver for even symbols and FIG. 8( b)illustrating results for odd symbols: it can be seen that the averagedistance at the interleaver output of sub-carriers that were adjacent atthe interleaver input is greater for an interleaver for odd symbols thanan interleaver for even symbols.

As will be understood, the amount of interleaver memory required toimplement a symbol interleaver is dependent on the number of datasymbols to be mapped onto the COFDM carrier symbols. Thus a 16 k modesymbol interleaver requires half the memory required to implement a 32 kmode symbol interleaver and similarly, the amount of memory required toimplement an 8 k symbol interleaver is half that required to implement a16 k interleaver. Therefore a transmitter or receiver which is arrangedto implement a symbol interleaver of a mode, which sets the maximumnumber of data symbols which can be carried per OFDM symbol, then thatreceiver or transmitter will include sufficient memory to implement twoodd interleaving processes for any other mode, which provides half orsmaller than half the number of sub-carriers per OFDM symbol in thatgiven maximum mode. For example a receiver or transmitter including a 32k interleaver will have enough memory to accommodate two 16 k oddinterleaving processes each with their own 16 k memory.

Therefore, in order to exploit the better performance of the oddinterleaving process, a symbol interleaver capable of accommodatingmultiple modulation modes can be arranged so that only an odd symbolinterleaving process is used if in a mode which comprises half or lessthan half of the number of sub-carriers in a maximum mode, whichrepresents the maximum number of sub-carriers per OFDM symbol. Thismaximum mode therefore sets the maximum memory size. For example, in atransmitter/receiver capable of the 32 k mode, when operating in a modewith fewer carriers (i.e. 16 k, 8 k, 4 k or 1 k) then rather thanemploying separate odd and even symbol interleaving processes, two oddinterleavers could be used.

An illustration is shown in FIG. 9 of an adaptation of the symbolinterleaver 33, which is shown in FIG. 3 when interleaving input datasymbols onto the sub-carriers of OFDM symbols in the odd interleavingmode only. The symbol interleaver 33.1 corresponds exactly to the symbolinterleaver 33 as shown in FIG. 3, except that the address generator 102is adapted to perform the odd interleaving process only. For the exampleshown in FIG. 9, the symbol interleaver 33.1 is operating in a modewhere the number of data symbols which can be carried per OFDM symbol isless than half of the maximum number which can be carried by an OFDMsymbol in an operating mode with the largest number of sub-carriers perOFDM symbol. As such, the symbol interleaver 33.1 has been arranged topartition the interleaver memory 100. For the present illustration shownin FIG. 9 the interleaver memory then 100 is divided into two parts 401,402. As an illustration of the symbol interleaver 33.1 operating in amode in which data symbols are mapped onto the OFDM symbols using theodd interleaving process, FIG. 9 provides an expanded view of each halfof the interleaver memory 401, 402. The expanded view provides anillustration of the odd interleaving mode as represented for thetransmitter side for four symbols A, B, C, D reproduced from FIG. 4.Thus as shown in FIG. 9, for successive sets of first and second datasymbols, the data symbols are written into the interleaver memory 401,402 in a sequential order and read out in a permuted order in accordancewith the addresses generated by the address generator 102 as previouslyexplained. Thus as illustrated in FIG. 9, since an odd interleavingprocess is being performed for successive sets of first and second setsof data symbols, the interleaver memory must be partitioned into twoparts. Symbols from a first set of data symbols are written into a firsthalf of the interleaver memory 401, and symbols from a second set ofdata symbols are written into a second part of the interleaver memory402. This is because the symbol interleaver is no longer able to reusethe same parts of the symbol interleaver memory as can be accommodatedwhen operating in an odd and even mode of interleaving.

A corresponding example of the interleaver in the receiver, whichappears in FIG. 7 but adapted to operate with an odd interleavingprocess only, is shown in FIG. 10. As shown in FIG. 10 the interleavermemory 540 is divided into two halves 410, 412 and the address generator542 is adapted to write data symbols into the interleaver memory andread data symbols from the interleaver memory into respective parts ofthe memory 410, 412 for successive sets of data symbols to implement anodd interleaving process only. Therefore, in correspondence withrepresentation shown in FIG. 9, FIG. 10 shows the mapping of theinterleaving process which is performed at the receiver and illustratedin FIG. 4 as an expanded view operating for both the first and secondhalves of the interleaving memory 410, 412. Thus a first set of datasymbols are written into a first part of the interleaver memory 410 in apermuted order defined in accordance with the addresses generated by theaddress generator 542 as illustrated by the order of writing in the datasymbols which provides a write sequence of 1, 3, 0, 2. As illustratedthe data symbols are then read out of the first part of the interleavermemory 410 in a sequential order thus recovering the original sequenceA, B, C, D.

Correspondingly, a second subsequent set of data symbols which arerecovered from a successive OFDM symbol are written into the second halfof the interleaver memory 412 in accordance with the addresses generatedby the address generator 542 in a permuted order and read out into theoutput data stream in a sequential order.

In one example the addresses generated for a first set of data symbolsto write into the first half of the interleaver memory 410 can be reusedto write a second subsequent set of data symbols into the interleavermemory 412. Correspondingly, the transmitter may also reuse addressesgenerated for one half of the interleaver for a first set of datasymbols for reading out a second set of data symbols which have beenwritten into the second half of the memory in sequential order.

Odd Interleaver with Offset

The performance of an interleaver, which uses two odd interleavers couldbe further improved by using a sequence of odd only interleavers ratherthan a single odd only interleaver, so that any bit of data input to theinterleave does not always modulate the same carrier in the OFDM symbol.

A sequence of odd only interleavers could be realised by either:

-   -   adding an offset to the interleaver address modulo the number of        data carriers, or    -   using a sequence of permutations in the interleaver        Adding an Offset

Adding an offset to the interleaver address modulo the number of datacarriers effectively shifts and wraps-round the OFDM symbol so that anybit of data input to the interleaver does not always modulate the samecarrier in the OFDM symbol. Thus the address generator, could optionallyinclude an offset generator, which generates an offset in an addressgenerated by the address generator on the output channel H(q).

The offset would change each symbol. For example, this offset couldprovide be a cyclic sequence. This cyclic sequence could be, forexample, of length 4 and could consist of, for example, prime numbers.For example, such a sequence could be:

0, 41, 97, 157

Furthermore, the offset may be a random sequence, which may be generatedby another address generator from a similar OFDM symbol interleaver ormay be generated by some other means.

Using a Sequence of Permutations

As shown in FIGS. 5, a control line 111 extends from the control unit ofthe address generator to the permutation circuit. As mentioned above, inone example the address generator can apply a different permutation codefrom a set of permutation codes for successive OFDM symbols. Using asequence of permutations in the interleaver address generator reduces alikelihood that any bit of data input to the interleaver does not alwaysmodulate the same sub-carrier in the OFDM symbol.

For example, this could be a cyclic sequence, so that a differentpermutation code in a set of permutation codes in a sequence is used forsuccessive OFDM symbols and then repeated. This cyclic sequence couldbe, for example, of length two or four. For the example of the 8 ksymbol interleaver a sequence of two permutation codes which are cycledthrough per OFDM symbol could be for example:

5 11 3 0 10 8 6 9 2 4 1 7 *

8 10 7 6 0 5 2 1 3 9 4 11

whereas a sequence of four permutation codes could be:

5 11 3 0 10 8 6 9 2 4 1 7 *

8 10 7 6 0 5 2 1 3 9 4 11

11 3 6 9 2 7 4 10 5 1 0 8

10 8 1 7 5 6 0 11 4 2 9 3

The switching of one permutation code to another could be effected inresponse to a change in the Odd/Even signal indicated on the controlchannel 108. In response the control unit 224 changes the permutationcode in the permutation code circuit 210 via the control line 111.

For the example of a 1 k symbol interleaver, two permutation codes couldbe:

4 3 2 1 0 5 6 7 8

3 2 5 0 1 4 7 8 6

whereas four permutation codes could be:

4 3 2 1 0 5 6 7 8

3 2 5 0 1 4 7 8 6

7 5 3 8 2 6 1 4 0

1 6 8 2 5 3 4 0 7

Other combinations of sequences may be possible for 2 k, 4 k and 16 kcarrier modes or indeed 0.5 k carrier mode. For example, the followingpermutation codes for each of the 0.5 k, 2 k, 4 k and 16 k provide goodde-correlation of symbols and can be used cyclically to generate theoffset to the address generated by an address generator for each of therespective modes:

2 k mode:

0 7 5 1 8 2 6 9 3 4 *

4 8 3 2 9 0 1 5 6 7

8 3 9 0 2 1 5 7 4 6

7 0 4 8 3 6 9 1 5 2

4 k mode:

7 10 5 8 1 2 4 9 0 3 6 **

6 2 7 10 8 0 3 4 1 9 5

9 5 4 2 3 10 1 0 6 8 7

1 4 10 3 9 7 2 6 5 0 8

16 k mode:

8 4 3 2 0 11 1 5 12 10 6 7 9

7 9 5 3 11 1 4 0 2 12 10 8 6

6 11 7 5 2 3 0 1 10 8 12 9 4

5 12 9 0 3 10 2 4 6 7 8 11 1

For the permutation codes indicated above, the first two could be usedin a two sequence cycle, whereas all four could be used for a foursequence cycle. In addition, some further sequences of four permutationcodes, which are cycled through to provide the offset in an addressgenerator to produce a good de-correlation in the interleaved symbols(some are common to the above) are provided below:

0.5 k mode:

3 7 4 6 1 2 0 5

4 2 5 7 3 0 1 6

5 3 6 0 4 1 2 7

6 1 0 5 2 7 4 3

2 k mode:

0 7 5 1 8 2 6 9 3 4 *

3 2 7 0 1 5 8 4 9 6

4 8 3 2 9 0 1 5 6 7

7 3 9 5 2 1 0 6 4 8

4 k mode:

7 10 5 8 1 2 4 9 0 3 6 **

6 2 7 10 8 0 3 4 1 9 5

10 3 4 1 2 7 0 6 8 5 9

0 8 9 5 10 4 6 3 2 1 7

8 k mode:

5 11 3 0 10 8 6 9 2 4 1 7*

10 8 5 4 2 9 1 0 6 7 3 11

11 6 9 8 4 7 2 1 0 10 5 3

8 3 11 7 9 1 5 6 4 0 2 10

*these are the permutations in the DVB-T standard

**these are the permutations in the DVB-H standard

Examples of address generators, and corresponding interleavers, for the2 k, 4 k and 8 k modes are disclosed in European patent applicationnumber 04251667.4, which corresponds to U.S. Patent ApplicationPublication No. 2008/298487, the contents of which are incorporatedherein by reference. An address generator for the 0.5 k mode aredisclosed in our co-pending UK patent application number 0722553.5,which corresponds to UK published application number GB 2454722. Variousmodifications may be made to the embodiments described above withoutdeparting from the scope of the present invention. In particular, theexample representation of the generator polynomial and the permutationorder which have been used to represent aspects of the invention are notintended to be limiting and extend to equivalent forms of the generatorpolynomial and the permutation order.

As will be appreciated the transmitter and receiver shown in FIGS. 1 and6 respectively are provided as illustrations only and are not intendedto be limiting. For example, it will be appreciated that the position ofthe symbol interleaver and the de-interleaver with respect, for exampleto the bit interleaver and the mapper can be changed. As will beappreciated the effect of the interleaver and de-interleaver isun-changed by its relative position, although the interleaver may beinterleaving I/Q symbols instead of v-bit vectors. A correspondingchange may be made in the receiver. Accordingly the interleaver andde-interleaver may be operating on different data types, and may bepositioned differently to the position described in the exampleembodiments.

According to one implementation of a receiver, a data processingapparatus is provided to map symbols received from a predeterminednumber of sub-carrier signals of an Orthogonal Frequency DivisionMultiplexed (OFDM) symbol into an output symbol stream.

As explained above the permutation codes and generator polynomial of theinterleaver, which has been described with reference to animplementation of a particular mode, can equally be applied to othermodes, by changing the predetermined maximum allowed address inaccordance with the number of sub-carriers for that mode.

As mentioned above, embodiments of the present invention findapplication with DVB standards such as DVB-T, DVB-T2 (published as EN302 755) and DVB-H (published as ETSI EN 302 304 V1.1.1 (2004-11)),which are incorporated herein by reference. For example embodiments ofthe present invention may be used in a transmitter or receiver operatingin accordance with the DVB-H standard, in hand-held mobile terminals.The mobile terminals may be integrated with mobile telephones (whethersecond, third or higher generation) or Personal Digital Assistants orTablet PCs for example. Such mobile terminals may be capable ofreceiving DVB-H or DVB-T/T2 compatible signals inside buildings or onthe move in for example cars or trains, even at high speeds. The mobileterminals may be, for example, powered by batteries, mains electricityor low voltage DC supply or powered from a car battery. Services thatmay be provided by DVB-H may include voice, messaging, internetbrowsing, radio, still and/or moving video images, television services,interactive services, video or near-video on demand and option. Theservices might operate in combination with one another. In otherexamples embodiments of the present invention finds application with theDVB-T2 standard as specified in accordance with ETSI standard EN 302755. In other examples embodiments of the present invention findapplication with the cable transmission standard known as DVB-C2.However, it will be appreciated that the present invention is notlimited to application with DVB and may be extended to other standardsfor transmission or reception, both fixed and mobile.

1. A data processing apparatus configured to map input symbols to becommunicated onto a predetermined number of sub-carrier signals ofOrthogonal Frequency Division Multiplexed (OFDM) symbols, the dataprocessing apparatus comprising: an interleaver configured to read-intoa memory the predetermined number of data symbols for mapping onto theOFDM sub-carrier signals, and to read-out of the memory the data symbolsfor the OFDM sub-carriers to effect the mapping, the read-out being in adifferent order than the read-in, the order being determined from a setof addresses, with the effect that the data symbols are interleaved onthe sub-carrier signals, and an address generator configured to generatethe set of addresses, an address being generated for each of the inputsymbols for mapping the input data symbol on to one of the sub-carriersignals, the address generator comprising: a linear feedback shiftregister including a predetermined number of register stages and beingconfigured to generate a pseudo-random bit sequence in accordance with agenerator polynomial, a permutation circuit configured to receive thecontent of the shift register stages and to permute the order of thebits present in the register stages in accordance with a permutationcode to form an address, and a control unit operable in combination withan address check circuit to re-generate an address when a generatedaddress exceeds a predetermined maximum valid address, wherein thepredetermined maximum valid address is approximately eight thousand, thelinear feedback shift register has twelve register stages with agenerator polynomial for the linear feedback shift register ofR′_(i)[11]=R′_(i-1)[0]⊕R′_(i-1)[1]⊕R′_(i-1)[4]⊕R′_(i-1)[6], and thepermutation code forms, with an additional bit, a thirteen bit address,wherein the permutation circuit is configured to change the permutationcode, which permutes the order of the bits of the register stages toform the addresses from one OFDM symbol to another.
 2. The dataprocessing apparatus as claimed in claim 1, wherein the permutationcircuit is configured to cycle through a sequence of differentpermutation codes for successive OFDM symbols.
 3. The data processingapparatus as claimed in claim 2, wherein one of the sequence ofdifferent permutation codes forms the thirteen bit address R_(i)[n] forthe i-th data symbol from the bit present in the n-th register stageR_(i)[n] in accordance with the permutation code defined by the table:R′_(i) bit positions 11 10 9 8 7 6 5 4 3 2 1 0 R_(i) bit positions 5 113 0 10 8 6 9 2 4 1 
 7.


4. The data processing apparatus as claimed in claim 2, wherein thesequence of permutation codes comprises two permutation codes, which areR′_(i) bit positions 11 10 9 8 7 6 5 4 3 2 1 0 R_(i) bit positions 5 113 0 10 8 6 9 2 4 1 7

and R′_(i) bit positions 11 10 9 8 7 6 5 4 3 2 1  0  R_(i) bit positions8 10 7 6 0 5 2 1 3 9 4
 11.


5. The data processing apparatus as claimed in claim 1, wherein thepredetermined maximum valid address is a value substantially between sixthousand and eight thousand one hundred and ninety two.
 6. The dataprocessing apparatus as claimed in claim 5, wherein the OFDM symbolincludes pilot sub-carriers, which are arranged to carry known symbols,and the predetermined maximum valid address depends on a number of thepilot sub-carrier symbols present in the OFDM symbol.
 7. The dataprocessing apparatus as claimed in claim 1, wherein the approximatelyeight thousand sub-carriers is provided by one of a plurality ofoperating modes in which the approximately eight thousand sub-carriersprovides half or less than half a maximum number of sub-carriers in theOFDM symbols of any of the operating modes, and the input data symbolsinclude first sets of input data symbols for mapping onto first OFDMsymbols and second sets of input data symbols for mapping onto secondOFDM symbols, and the data processing apparatus is configured tointerleave the input data symbols from both first and second sets inaccordance with an odd interleaving process, the odd interleavingprocess including: writing the first sets of input data symbols into afirst part of the interleaver memory in accordance with a sequentialorder of the first sets of input data symbols, reading out the firstsets of input data symbols from the first part of the interleaver memoryon to the sub-carrier signals of the first OFDM symbols in accordancewith an order defined by the set of addresses generated with one of thepermutation codes of the sequence, writing the second set of input datasymbols into a second part of the interleaver memory in accordance witha sequential order of the second sets of input data symbols, and readingout the second sets of input data symbols from the second part of theinterleaver memory on to the sub-carrier signals of the second OFDMsymbols in accordance with an order defined by the set of addressesgenerated with another of the permutation codes of the sequence.
 8. Atransmitter for transmitting input data symbols using OrthogonalFrequency Division Multiplexing (OFDM), the transmitter including a dataprocessing apparatus for mapping the input data symbols on apredetermined number of sub-carrier signals of OFDM symbols, the dataprocessing apparatus comprising: an interleaver configured to read-intoa memory the predetermined number of data symbols for mapping onto theOFDM sub-carrier signals, and to read-out of the memory the data symbolsfor the OFDM sub-carriers to effect the mapping, the read-out being in adifferent order than the read-in, the order being determined from a setof addresses, with the effect that the data symbols are interleaved onthe sub-carrier signals, and an address generator configured to generatethe set of addresses, an address being generated for each of the inputsymbols for mapping on to one of the sub-carrier signals, the addressgenerator comprising: a linear feedback shift register including apredetermined number of register stages and being configured to generatea pseudo-random bit sequence in accordance with a generator polynomial,a permutation circuit configured to receive the content of the shiftregister stages and to permute the order of the bits present in theregister stages in accordance with a permutation code to form anaddress, and a control unit operable in combination with an addresscheck circuit to re-generate an address when a generated address exceedsa predetermined maximum valid address, wherein the predetermined maximumvalid address is approximately eight thousand, the linear feedback shiftregister has twelve register stages with a generator polynomial for thelinear feedback shift register ofR′_(i)[11]=R′_(i-1)[0]⊕R′_(i-1)[1]⊕R′_(i-1)[4]⊕R′_(i-1)[6], and thepermutation code forms, with an additional bit, a thirteen bit address,characterised in that the permutation circuit is configured to changethe permutation code, which permutes the order of the bits of theregister stages to form the addresses from one OFDM symbol to another.9. The transmitter as claimed in claim 8, wherein the transmitter isconfigured to transmit data in accordance with a Digital VideoBroadcasting standard, including the Digital VideoBroadcasting-Terrestrial standard, the Digital VideoBroadcasting-Handheld standard or the Digital VideoBroadcasting-Terrestrial2 standard.
 10. A method of mapping inputsymbols to be communicated onto a predetermined number of sub-carriersignals of an Orthogonal Frequency Division Multiplexed (OFDM) symbols,the method comprising: reading-into a memory the predetermined number ofdata symbols for mapping onto the OFDM sub-carrier signals, reading-outof the memory the data symbols for the OFDM sub-carriers to effect themapping, the read-out being in a different order than the read-in, theorder being determined from a set of addresses, with the effect that thedata symbols are interleaved on the sub-carrier signals, and generatingthe set of addresses, an address being generated for each of the inputsymbols for mapping the input data symbol on to one of the sub-carriersignals, the generating the set of addresses comprising: using a linearfeedback shift register including a predetermined number of registerstages to generate a pseudo-random bit sequence in accordance with agenerator polynomial, using a permutation circuit configured to receivethe content of the shift register stages to permute the order of thebits present in the register stages in accordance with a permutationcode to form an address, and re-generating an address when a generatedaddress exceeds a predetermined maximum valid address, wherein thepredetermined maximum valid address is approximately eight thousand, thelinear feedback shift register has twelve register stages with agenerator polynomial for the linear feedback shift register ofR′_(i)[11]=R′_(i-1)[0]⊕R′_(i-1)[1]⊕R′_(i-1)[4]⊕R′_(i-1)[6], and thepermutation code forms, with an additional bit, a thirteen bit address,characterised by changing the permutation code, which permutes the orderof the bits of the register stages to form the set of addresses from oneOFDM symbol to another.
 11. The method as claimed in claim 10, whereinthe changing the permutation code includes cycling through a sequence ofdifferent permutation codes for successive OFDM symbols.
 12. The methodas claimed in claim 11, wherein one of the sequence of differentpermutation codes forms the thirteen bit address R_(i)[n] for the i-thdata symbol from the bit present in the n-th register stage R′_(i)[n] inaccordance with the permutation code is defined by the table: R′_(i) bitpositions 11 10 9 8 7 6 5 4 3 2 1 0 R_(i) bit positions 5 11 3 0 10 8 69 2 4 1 
 7.


13. The method as claimed in claim 11, wherein the sequence ofpermutation codes comprises two permutation codes, which are R′_(i) bitpositions 11 10 9 8 7 6 5 4 3 2 1 0 R_(i) bit positions 5 11 3 0 10 8 69 2 4 1 7

and R′_(i) bit positions 11 10 9 8 7 6 5 4 3 2 1  0  R_(i) bit positions8 10 7 6 0 5 2 1 3 9 4
 11.


14. The method as claimed in claim 10, wherein the predetermined maximumvalid address is a value substantially between six thousand and eightthousand one hundred and ninety two.
 15. The method as claimed in claim14, wherein the OFDM symbol includes pilot sub-carriers, which arearranged to carry known symbols, and the predetermined maximum validaddress depends on a number of the pilot sub-carrier symbols present inthe OFDM symbol.
 16. The method as claimed claim 10, wherein theapproximately eight thousand sub-carriers is provided by one of aplurality of operating modes in which the approximately eight thousandsub-carriers provide half or less than half a maximum number ofsub-carriers in the OFDM symbols of any of the operating modes, themethod comprising: dividing the input data symbols include first sets ofinput data symbols for mapping onto first OFDM symbols and second setsof input data symbols for mapping onto second OFDM symbols, andinterleaving the input data symbols from both first and second sets inaccordance with an odd interleaving process comprising: writing thefirst sets of input data symbols into a first part of the interleavermemory in accordance with a sequential order of the first sets of inputdata symbols, reading out the first sets of input data symbols from thefirst part of the interleaver memory on to the sub-carrier signals ofthe first OFDM symbols in accordance with an order defined by the set ofaddresses generated with one of the permutation codes of the sequence,writing the second set of input data symbols into a second part of theinterleaver memory in accordance with a sequential order of the secondsets of input data symbols, and reading out the second sets of inputdata symbols from the second part of the interleaver memory on to thesub-carrier signals of the second OFDM symbols in accordance with anorder defined by the set of addresses generated with another of thepermutation codes of the sequence.
 17. A method of transmitting datasymbols via a predetermined number of sub-carrier signals of anOrthogonal Frequency Division Multiplexed (OFDM) symbol, the methodcomprising: reading-into a memory the predetermined number of datasymbols for mapping onto the OFDM sub-carrier signals, reading-out ofthe memory the data symbols for transmission on the OFDM sub-carriers toeffect the mapping, the read-out being in a different order than theread-in, the order being determined from a set of addresses, with theeffect that the data symbols are interleaved on the sub-carrier signals,and generating the set of addresses, an address being generated for eachof the input symbols for mapping on to one of the sub-carrier signals,the generating the set of addresses comprising: using a linear feedbackshift register including a predetermined number of register stages togenerate a pseudo-random bit sequence in accordance with a generatorpolynomial, using a permutation circuit configured to receive thecontent of the shift register stages to permute the order of the bitspresent in the register stages in accordance with a permutation code toform an address, and re-generating an address when a generated addressexceeds a predetermined maximum valid address, wherein the predeterminedmaximum valid address is approximately eight thousand, the linearfeedback shift register has twelve register stages with a generatorpolynomial for the linear feedback shift register ofR′_(i)[11]=R′_(i-1)[0]⊕R′_(i-1)[1]⊕R′_(i-1)[4]⊕R′_(i-1)[6], and thepermutation code forms, with an additional bit, a thirteen bit address,characterised by changing the permutation code, which permutes the orderof the bits of the register stages to form the set of addresses from oneOFDM symbol to another.
 18. The method of transmitting as claimed inclaim 17, wherein the transmitting includes transmitting in accordancewith a Digital Video Broadcasting standard including the Digital VideoBroadcasting-Terrestrial standard, the Digital VideoBroadcasting-Handheld standard or the Digital VideoBroadcasting-Terrestrial2 standard.
 19. An address generator for usewith transmission of data symbols interleaved onto sub-carriers of anOrthogonal Frequency Division Multiplexed symbol, the address generatorbeing configured to generate a set of addresses, each address beinggenerated for each of the data symbols for mapping the data symbols onto one of the sub-carrier signals, the address generator comprising: alinear feedback shift register including a predetermined number ofregister stages and being configured to generate a pseudo-random bitsequence in accordance with a generator polynomial, a permutationcircuit configured to receive the content of the shift register stagesand to permute the order of the bits present in the register stages inaccordance with a permutation code to form an address, and a controlunit configured in combination with an address check circuit tore-generate an address when a generated address exceeds a predeterminedmaximum valid address, wherein the predetermined maximum valid addressis approximately eight thousand, the linear feedback shift register hastwelve register stages with a generator polynomial for the linearfeedback shift register ofR′_(i)[11]=R′_(i-1)[0]⊕R′_(i-1)[1]⊕R′_(i-1)[4]⊕R′_(i-1)[6], and thepermutation order forms, with an additional bit, a thirteen bit address,wherein the permutation circuit is configured to change the permutationcode, which permutes the order of the bits of the register stages toform the set of addresses from one OFDM symbol to another.
 20. Theaddress generator as claimed in claim 19, wherein the permutationcircuit is configured to cycle through a sequence of differentpermutation codes for successive OFDM symbols.
 21. The address generatoras claimed in claim 20, wherein one of the sequence of differentpermutation codes forms the thirteen bit address R_(i)[n] for the i-thdata symbol from the bit present in the n-th register stage R′_(i)[n] inaccordance with the permutation code is defined by the table: R′_(i) bitpositions 11 10 9 8 7 6 5 4 3 2 1 0 R_(i) bit positions 5 11 3 0 10 8 69 2 4 1 
 7.


22. The address generator as claimed in claim 20, wherein the sequenceof permutation codes comprises two permutation codes, which are R′_(i)bit positions 11 10 9 8 7 6 5 4 3 2 1 0 R_(i) bit positions 5 11 3 0 108 6 9 2 4 1 7

and R′_(i) bit positions 11 10 9 8 7 6 5 4 3 2 1  0  R_(i) bit positions8 10 7 6 0 5 2 1 3 9 4
 11.


23. A data processing apparatus configured to map input data symbols tobe communicated onto a predetermined number of sub-carrier signals ofOrthogonal Frequency Division Multiplexed OFDM symbols, thepredetermined number of sub-carrier signals being determined inaccordance with one of a plurality of operating modes and the input datasymbols including first sets of input data symbols for mapping ontofirst OFDM symbols and second sets of input data symbols for mappingonto second OFDM symbols, the data processing apparatus comprising: aninterleaver configured to read-into a memory the predetermined number ofdata symbols for mapping onto the OFDM sub-carrier signals, and toread-out of the memory the data symbols for the OFDM sub-carriers toeffect the mapping, the read-out being in a different order than theread-in, the order being determined from a set of addresses, with theeffect that the data symbols are interleaved on the sub-carrier signals,and an address generator configured to generate the set of addresses, anaddress being generated for each of the input symbols for mapping theinput data symbols onto one of the sub-carrier signals, the addressgenerator comprising: a linear feedback shift register including apredetermined number of register stages and being configured to generatea pseudo-random bit sequence in accordance with a generator polynomial,a permutation circuit configured to receive the content of the shiftregister stages and to permute the order of the bits present in theregister stages in accordance with a permutation code to form an addressof one of the OFDM sub-carriers, and a control unit configured incombination with an address check circuit to re-generate an address whena generated address exceeds a predetermined maximum valid address,wherein one of a plurality of operating modes provides approximatelyeight thousand sub-carriers per OFDM symbol, the approximately eightthousand sub-carriers providing half or less than half a maximum numberof sub-carriers in the OFDM symbols of any of the operating modes, thepredetermined maximum valid address is approximately eight thousand, thelinear feedback shift register has twelve register stages with agenerator polynomial for the linear feedback shift register ofR′_(i)[11]=R′_(i-1)[0]⊕R′_(i-1)[1]⊕R′_(i-1)[4]⊕R′_(i-1)[6], and thepermutation code forms, with an additional bit, a thirteen bit address,and the data processing apparatus is configured to interleave the inputdata symbols from both first and second sets in accordance with an oddinterleaving process, the odd interleaving process including: writingthe first sets of input data symbols into a first part of theinterleaver memory in accordance with a sequential order of the firstsets of input data symbols, reading out the first sets of input datasymbols from the first part of the interleaver memory on to thesub-carrier signals of the first OFDM symbols in accordance with anorder defined by the set of addresses, writing the second set of inputdata symbols into a second part of the interleaver memory in accordancewith a sequential order of the second sets of input data symbols, andreading out the second sets of input data symbols from the second partof the interleaver memory on to the sub-carrier signals of the secondOFDM symbols in accordance with an order defined by the set ofaddresses.
 24. The data processing apparatus as claimed in claim 23,wherein the permutation code forms the thirteen bit address R_(i)[n] forthe i-th data symbol from the bit present in the n-th register stageR′_(i)[n] in accordance with the permutation code defined by the table:R′_(i) bit positions 11 10 9 8 7 6 5 4 3 2 1 0 R_(i) bit positions 5 113 0 10 8 6 9 2 4 1 
 7.


25. A method of mapping input data symbols to be communicated onto apredetermined number of sub-carrier signals of Orthogonal FrequencyDivision Multiplexed OFDM symbols, the predetermined number ofsub-carrier signals being determined in accordance with one of aplurality of operating modes and the input data symbols including firstsets of input data symbols for mapping onto first OFDM symbols andsecond sets of input data symbols for second OFDM symbols, the methodcomprising: reading-into a memory the predetermined number of datasymbols for mapping onto the OFDM sub-carrier signals, reading-out ofthe memory the data symbols for the OFDM sub-carriers to effect themapping, the read-out being in a different order than the read-in, theorder being determined from a set of addresses, with the effect that thedata symbols are interleaved on the sub-carrier signals, and generatingthe set of addresses, an address being generated for each of the inputsymbols for mapping the input data symbol onto one of the sub-carriersignals, the generating the set of addresses comprising: using a linearfeedback shift register including a predetermined number of registerstages to generate a pseudo-random bit sequence in accordance with agenerator polynomial, using a permutation circuit configured to receivethe content of the shift register stages to permute the order of thebits present in the register stages in accordance with a permutationcode to form an address, and re-generating an address when a generatedaddress exceeds a predetermined maximum valid address, wherein one of aplurality of operating modes provide approximately eight thousandsub-carriers, the approximately eight thousand sub-carriers providinghalf or less than half a maximum number of sub-carriers in the OFDMsymbols of any of the operating modes, the predetermined maximum validaddress is approximately eight thousand, the linear feedback shiftregister has twelve register stages with a generator polynomial for thelinear feedback shift register ofR′_(i)[11]=R′_(i-1)[0]⊕R′_(i-1)[1]⊕R′_(i-1)[4]⊕R′_(i-1)[6], and thepermutation code forms, with an additional bit, a thirteen bit address,and interleaving the input data symbols from both first and second setsin accordance with an odd interleaving process comprising: writing thefirst sets of input data symbols into a first part of the interleavermemory in accordance with a sequential order of the first sets of inputdata symbols, reading out the first sets of input data symbols from thefirst part of the interleaver memory on to the sub-carrier signals ofthe first OFDM symbols in accordance with an order defined by the set ofaddresses, writing the second set of input data symbols into a secondpart of the interleaver memory in accordance with a sequential order ofthe second sets of input data symbols, and reading out the second setsof input data symbols from the second part of the interleaver memory onto the sub-carrier signals of the second OFDM symbols in accordance withan order defined by the set of addresses.
 26. The method as claimed inclaim 25, wherein the permutation code forms the thirteen bit addressR_(i)[n] for the i-th data symbol from the bit present in the n-thregister stage R′_(i)[n] in accordance with the permutation code isdefined by the table: R′_(i) bit positions 11 10 9 8 7 6 5 4 3 2 1 0R_(i) bit positions 5 11 3 0 10 8 6 9 2 4 1 
 7.


27. A transmitter for transmitting input data symbols to be communicatedonto a predetermined number of sub-carrier signals of OrthogonalFrequency Division Multiplexed (OFDM) symbols, the transmittercomprising: an interleaver configured to read-into a memory thepredetermined number of data symbols for mapping onto the OFDMsub-carrier signals, and to read-out of the memory the data symbols forthe OFDM sub-carriers to effect the mapping, the read-out being in adifferent order than the read-in, the order being determined from a setof addresses, with the effect that the data symbols are interleaved onthe sub-carrier signals, and an address generator configured to providethe set of addresses, an address being generated for each of the inputsymbols for mapping the input data symbols on to one of the sub-carriersignals, the address generator comprising a memory containing the set ofaddresses, the set of addresses having been generated and stored in thememory using: a linear feedback shift register including a predeterminednumber of register stages and being configured to generate apseudo-random bit sequence in accordance with a generator polynomial, apermutation circuit configured to receive the content of the shiftregister stages and to permute the order of the bits present in theregister stages in accordance with a permutation code to form anaddress, and a control unit configured in combination with an addresscheck circuit to re-generate an address when a generated address exceedsa predetermined maximum valid address, wherein the predetermined maximumvalid address is approximately eight thousand, the linear feedback shiftregister has twelve register stages with a generator polynomial for thelinear feedback shift register ofR′_(i)[11]=R′_(i-1)[0]⊕R′_(i-1)[1]⊕R′_(i-1)[4]⊕R′_(i-1)[6], and thepermutation code forms, with an additional bit, a thirteen bit address,characterised in that the permutation circuit is configured to changethe permutation code, which permutes the order of the bits of theregister stages to form the addresses from one OFDM symbol to another.28. A method of transmitting input data symbols to be communicated ontoa predetermined number of sub-carrier signals of an Orthogonal FrequencyDivision Multiplexed (OFDM) symbols, the method comprising: reading-intoa memory the predetermined number of data symbols for mapping onto theOFDM sub-carrier signals, reading-out of the memory the data symbols forthe OFDM sub-carriers to effect the mapping, the read-out being in adifferent order than the read-in, the order being determined from a setof addresses, with the effect that the data symbols are interleaved onthe sub-carrier signals, and providing the set of addresses, an addressbeing provided for each of the input symbols for mapping the input datasymbol on to one of the sub-carrier signals, the providing the set ofaddresses comprising reading the set of addresses from a memorycontaining the set of addresses, the set of addresses having beengenerated and stored in the memory by: using a linear feedback shiftregister including a predetermined number of register stages to generatea pseudo-random bit sequence in accordance with a generator polynomial,using a permutation circuit configured to receive the content of theshift register stages to permute the order of the bits present in theregister stages in accordance with a permutation code to form anaddress, and re-generating an address when a generated address exceeds apredetermined maximum valid address, wherein the predetermined maximumvalid address is approximately eight thousand, the linear feedback shiftregister has twelve register stages with a generator polynomial for thelinear feedback shift register ofR′_(i)[11]=R′_(i-1)[0]⊕R′_(i-1)[1]⊕R′_(i-1)[4]⊕R′_(i-1)[6], and thepermutation code forms, with an additional bit, a thirteen bit address,characterised by changing the permutation code, which permutes the orderof the bits of the register stages to form the set of addresses from oneOFDM symbol to another.
 29. A transmitter for transmitting input datasymbols using a predetermined number of sub-carrier signals ofOrthogonal Frequency Division Multiplexed OFDM symbols, thepredetermined number of sub-carrier signals being determined inaccordance with one of a plurality of operating modes and the input datasymbols including first sets of input data symbols for mapping ontofirst OFDM symbols and second sets of input data symbols for mappingonto second OFDM symbols, the data processing apparatus comprising: aninterleaver configured to read-into a memory the predetermined number ofdata symbols for mapping onto the OFDM sub-carrier signals, and toread-out of the memory the data symbols for the OFDM sub-carriers toeffect the mapping, the read-out being in a different order than theread-in, the order being determined from a set of addresses, with theeffect that the data symbols are interleaved on the sub-carrier signals,and an address generator configured to provide the set of addresses, anaddress being generated for each of the input symbols for mapping theinput data symbols on to one of the sub-carrier signals, the addressgenerator comprising a memory containing the set of addresses, the setof addresses having been generated and stored in the memory using: alinear feedback shift register including a predetermined number ofregister stages and being configured to generate a pseudo-random bitsequence in accordance with a generator polynomial, a permutationcircuit configured to receive the content of the shift register stagesand to permute the order of the bits present in the register stages inaccordance with a permutation code to form an address of one of the OFDMsub-carriers, and a control unit configured in combination with anaddress check circuit to re-generate an address when a generated addressexceeds a predetermined maximum valid address, wherein one of aplurality of operating modes provides approximately eight thousandsub-carriers per OFDM symbol, the approximately eight thousandsub-carriers providing half or less than half a maximum number ofsub-carriers in the OFDM symbols of any of the operating modes, thepredetermined maximum valid address is approximately eight thousand, thelinear feedback shift register has twelve register stages with agenerator polynomial for the linear feedback shift register ofR′_(i)[11]=R′_(i-1)[0]⊕R′_(i-1)[1]⊕R′_(i-1)[4]⊕R′_(i-1)[6], and thepermutation code forms, with an additional bit, a thirteen bit address,and the data processing apparatus is adapted to interleave the inputdata symbols from both first and second sets in accordance with an oddinterleaving process, the odd interleaving process including: writingthe first sets of input data symbols into a first part of theinterleaver memory in accordance with a sequential order of the firstsets of input data symbols, reading out the first sets of input datasymbols from the first part of the interleaver memory on to thesub-carrier signals of the first OFDM symbols in accordance with anorder defined by the set of addresses, writing the second set of inputdata symbols into a second part of the interleaver memory in accordancewith a sequential order of the second sets of input data symbols, andreading out the second sets of input data symbols from the second partof the interleaver memory on to the sub-carrier signals of the secondOFDM symbols in accordance with an order defined by the set ofaddresses.
 30. A method of transmitting input data symbols using apredetermined number of sub-carrier signals of Orthogonal FrequencyDivision Multiplexed OFDM symbols, the predetermined number ofsub-carrier signals being determined in accordance with one of aplurality of operating modes and the input data symbols including firstsets of input data symbols for mapping onto first OFDM symbols andsecond sets of input data symbols for second OFDM symbols, the methodcomprising: reading-into a memory the predetermined number of datasymbols for mapping onto the OFDM sub-carrier signals, reading-out ofthe memory the data symbols for the OFDM sub-carriers to effect themapping, the read-out being in a different order than the read-in, theorder being determined from a set of addresses, with the effect that thedata symbols are interleaved on the sub-carrier signals, and providingthe set of addresses, an address being provided for each of the inputsymbols for mapping the input data symbol on to one of the sub-carriersignals, the providing the set of addresses comprising reading the setof addresses from a memory containing the set of addresses, the set ofaddresses having been generated and stored in the memory by: using alinear feedback shift register including a predetermined number ofregister stages to generate a pseudo-random bit sequence in accordancewith a generator polynomial, using a permutation circuit configured toreceive the content of the shift register stages to permute the order ofthe bits present in the register stages in accordance with a permutationcode to form an address, and re-generating an address when a generatedaddress exceeds a predetermined maximum valid address, wherein one of aplurality of operating modes provide approximately eight thousandsub-carriers, the approximately eight thousand sub-carriers providinghalf or less than half a maximum number of sub-carriers in the OFDMsymbols of any of the operating modes, the predetermined maximum validaddress is approximately eight thousand, the linear feedback shiftregister has twelve register stages with a generator polynomial for thelinear feedback shift register ofR′_(i)[11]=R′_(i-1)[0]⊕R′_(i-1)[1]⊕R′_(i-1)[4]⊕R′_(i-1)[6], and thepermutation code forms, with an additional bit, a thirteen bit address,and interleaving the input data symbols from both first and second setsin accordance with an odd interleaving process comprising: writing thefirst sets of input data symbols into a first part of the interleavermemory in accordance with a sequential order of the first sets of inputdata symbols, reading out the first sets of input data symbols from thefirst part of the interleaver memory on to the sub-carrier signals ofthe first OFDM symbols in accordance with an order defined by the set ofaddresses, writing the second set of input data symbols into a secondpart of the interleaver memory in accordance with a sequential order ofthe second sets of input data symbols, and reading out the second setsof input data symbols from the second part of the interleaver memory onto the sub-carrier signals of the second OFDM symbols in accordance withan order defined by the set of addresses.